Material for lithium secondary battery of high performance

ABSTRACT

Provided is a cathode active material containing a Ni-based lithium mixed transition metal oxide. More specifically, the cathode active material comprises the lithium mixed transition metal oxide having a composition represented by Formula I of Li x M y O 2  wherein M, x and y are as defined in the specification, which is prepared by a solid-state reaction of Li 2 CO 3  with a mixed transition metal precursor under an oxygen-deficient atmosphere, and has a Li 2 CO 3  content of less than 0.07% by weight of the cathode active material as determined by pH titration. The cathode active material in accordance with the present invention and substantially free of water-soluble bases such as lithium carbonates and lithium sulfates and therefore has excellent high-temperature and storage stabilities and a stable crystal structure. A secondary battery comprising such a cathode active material exhibits a high capacity and excellent characteristics, and can be produced by an environmentally friendly method with low production costs and high production efficiency.

FIELD OF THE INVENTION

The present invention relates to a cathode active material containing aNi-based lithium mixed transition metal oxide. More specifically, thepresent invention relates to a cathode active material which comprises alithium mixed transition metal oxide having a given composition, inwhich the lithium mixed transition metal oxide is prepared by asolid-state reaction of Li₂CO₃ with a mixed transition metal precursorunder an oxygen-deficient atmosphere, and has a Li₂CO₃ content of lessthan 0.07% by weight of the cathode active material as determined by pHtitration.

BACKGROUND OF THE INVENTION

Technological development and increased demand for mobile equipment haveled to a rapid increase in the demand for secondary batteries as anenergy source. Among other things, lithium secondary batteries having ahigh-energy density and voltage, a long cycle lifespan and a lowself-discharge rate are commercially available and widely used.

As cathode active materials for the lithium secondary batteries,lithium-containing cobalt oxide (LiCoO₂) is largely used. In addition,consideration has been made to using lithium-containing manganese oxidessuch as LiMnO₂ having a layered crystal structure and LiMn₂O₄ having aspinel crystal structure, and lithium-containing nickel oxides (LiNiO₂).

Of the aforementioned cathode active materials, LiCoO₂ is currentlywidely used due to superior general properties including excellent cyclecharacteristics, but suffers from low safety, expensiveness due tofinite resources of cobalt as a raw material, and limitations inpractical and mass application thereof as a power source for electricvehicles (EVs) and the like.

Lithium manganese oxides, such as LiMnO₂ and LiMn₂O₄, are abundantresources as raw materials and advantageously employenvironmentally-friendly manganese, and therefore have attracted a greatdeal of attention as a cathode active material capable of substitutingLiCoO₂. However, these lithium manganese oxides suffer from shortcomingssuch as low capacity and poor cycle characteristics.

Whereas, lithium/nickel-based oxides including LiNiO₂ are inexpensive ascompared to the aforementioned cobalt-based oxides and exhibit a highdischarge capacity upon charging to 4.3 V. The reversible capacity ofdoped LiNiO₂ approximates about 200 mAh/g which exceeds the capacity ofLiCoO₂ (about 165 mAh/g). Therefore, despite a slightly lower averagedischarge voltage and a slightly lower volumetric density, commercialbatteries comprising LiNiO₂ as the cathode active material exhibit animproved energy density. To this end, a great deal of intensive researchis being actively undertaken on the feasibility of applications of suchnickel-based cathode active materials for the development ofhigh-capacity batteries. However, the LiNiO₂-based cathode activematerials suffer from some limitations in practical application thereof,due to the following problems.

First, LiNiO₂-based oxides undergo sharp phase transition of the crystalstructure with volumetric changes accompanied by repeatedcharge/discharge cycling, and thereby may suffer from cracking ofparticles or formation of voids in grain boundaries. Consequently,intercalation/deintercalation of lithium ions may be hindered toincrease the polarization resistance, thereby resulting in deteriorationof the charge/discharge performance. In order to prevent such problems,conventional prior arts attempted to prepare a LiNiO₂-based oxide byadding an excess of a Li source and reacting reaction components underan oxygen atmosphere. However, the thus-prepared cathode activematerial, under the charged state, undergoes structural swelling anddestabilization due to the repulsive force between oxygen atoms, andsuffers from problems of severe deterioration in cycle characteristicsdue to repeated charge/discharge cycles.

Second, LiNiO₂ has shortcomings associated with the evolution of excessgas during storage or cycling. That is, in order to smoothly form thecrystal structure, an excess of a Li source is added duringmanufacturing of the LiNiO₂-based oxide, followed by heat treatment. Asa result, water-soluble bases including Li₂CO₃ and LiOH reactionresidues remain between primary particles and thereby they decompose orreact with electrolytes to thereby produce CO₂ gas, upon charging.Further, LiNiO₂ particles have an agglomerate secondary particlestructure in which primary particles are agglomerated to form secondaryparticles and consequently a contact area with the electrolyte furtherincreases to result in severe evolution of CO₂ gas, which in turnunfortunately leads to the occurrence of battery swelling anddeterioration of desirable high-temperature safety.

Third, LiNiO₂ suffers from a sharp decrease in the chemical resistanceof a surface thereof upon exposure to air and moisture, and the gelationof slurries by polymerization of an N-methyl pyrrolidone/poly(vinylidenefluoride) (NMP-PVDF) slurry due to a high pH value. These properties ofLiNiO₂ cause severe processing problems during battery production.

Fourth, high-quality LiNiO₂ cannot be produced by a simple solid-statereaction as is used in the production of LiCoO₂, and LiNiMO₂ cathodeactive materials containing an essential dopant cobalt and furtherdopants manganese and aluminum are produced by reacting a lithium sourcesuch as LiOH.H₂O with a mixed transition metal hydroxide under an oxygenor syngas atmosphere (i.e., a CO₂-deficient atmosphere), whichconsequently increases production costs. Further, when an additionalstep, such as intermediary washing or coating, is included to removeimpurities in the production of LiNiO₂, this leads to a further increasein production costs.

Many prior arts focus on improving properties of LiNiO₂-based cathodeactive materials and processes to prepare LiNiO₂. However, variousproblems, such as high production costs, swelling due to gas evolutionin the fabricated batteries, poor chemical stability, high pH and thelike, have not been sufficiently solved. A few examples will beillustrated hereinafter.

U.S. Pat. No. 6,040,090 (T. Sunagawa et al., Sanyo) discloses a widerange of compositions including nickel-based and high-Ni LiMO₂, thematerials having high crystallinity and being used in Li-ion batteriesin ethylene carbonate (EC) containing an electrolyte. Samples wereprepared on a small scale, using LiOH.H₂O as a lithium source. Thesamples were prepared in a flow of synthetic air composed of a mixtureof oxygen and nitrogen, free of CO₂.

U.S. Pat. No. 5,264,201 (J. R. Dahn et al.) discloses a doped LiNiO₂substantially free of lithium hydroxides and lithium carbonates. Forthis purpose, lithium hydroxide and LiOH.H₂O as a lithium source areemployed and heat treatment is performed under an oxygen atmosphere freeof CO₂, additionally with a low content of H₂O. An excess of lithium“evaporates”; however, “evaporation” is a lab-scale effect and not anoption for large-scale preparation. That is, when applied to alarge-scale production process, it becomes difficult to evaporate excesslithium, thereby resulting in problems associated with the formation oflithium hydroxides and lithium carbonates.

U.S. Pat. No. 5,370,948 (M. Hasegawa et al., Matsushita) discloses aprocess for the production of Mn-doped LiNi_(1-x)Mn_(x)O₂ (x<0.45),wherein the manganese source is manganese nitrate, and the lithiumsource is either lithium hydroxide or lithium nitrate.

U.S. Pat. No. 5,393,622 (Y. Nitta et al., Matsushita) discloses aprocess to prepare LiNi_(1-x)Mn_(x)O₂ by a two-step heating, involvingpre-drying, cooking and the final heating. The final heating is done inan oxidizing gas such as air or oxygen. This patent focuses on oxygen.The disclosed method uses a very low temperature of 550 to 650° C. forcooking, and less than 800° C. for sintering. At higher temperatures,samples deteriorate dramatically. Excess lithium is used such that thefinal samples contain a large amount of water-soluble base (i.e.,lithium compounds). According to the research performed by the inventorsof the present invention, the observed deterioration is attributable tothe presence of lithium salts as impurities which melt at about 700 toabout 800° C., thereby detaching the crystallites.

WO 9940029 A1 (M. Benz et al., H. C. Stack) describes a complicatedpreparation method very different from that disclosed in the presentinvention. This preparation method involves the use of lithium nitratesand lithium hydroxides and recovering the evolved noxious gasses. Thesintering temperature never exceeds 800° C. and typically is far lower.

U.S. Pat. No. 4,980,080 (Lecerf, SAFT) describes a process to prepareLiNiO₂-based cathodes from lithium hydroxides and metal oxides attemperatures below 800° C.

In prior arts including the above, LiNiO₂-based cathode active materialsare generally prepared by high cost processes, in a specific reactionatmosphere, especially in a flow of synthetic gas such as oxygen orsynthetic air, free of CO₂, and using LiOH.H₂O, Li nitrate, Li acetate,etc., but not the inexpensive, easily manageable Li₂CO₃. Furthermore,the final cathode active materials have a high content of soluble bases,originating from carbonate impurities present in the precursors, whichremain in the final cathode because of the thermodynamic limitation.Further, the crystal structure of the final cathode active materials perse is basically unstable even when the final cathode active materialsare substantially free of soluble bases. Consequently, upon exposure toair containing moisture or carbon dioxide during storage of the activematerials, lithium is released to surfaces from the crystal structureand reacts with air to thereby result in continuous formation of solublebases.

Meanwhile, Japanese Unexamined Patent Publication Nos. 2004-281253,2005-150057 and 2005-310744 disclose oxides having a composition formulaof Li_(a)Mn_(x)Ni_(y)M_(z)O₂ (M=Co or Al, 1≦a≦1.2, 0≦x≦0.65, 0.35≦y≦1,0≦z≦0.65, and x+y+z=1). These inventions provide a method of preparingthe oxide involving mixing each transition metal precursor with alithium compound, grinding, drying and sintering the mixture, andre-grinding the sintered composite oxide by ball milling, followed byheat treatment. In addition, working examples disclosed in the aboveprior art employ substantially only LiOH as a lithium source. Further,it was found through various experiments conducted by the inventors ofthe present invention that the aforesaid prior art composite oxidesuffers from significant problems associated with a high-temperaturesafety, due to production of large amounts of impurities such as Li₂CO₃.

Alternatively, encapsulation of high Ni—LiNiO₂ by SiO_(x) protectivecoating has been proposed (H. Omanda, T. Brousse, C. Marhic, and D. M.Schleich, J. Electrochem. Soc. 2004, 151, A922.), but the resultingelectrochemical properties are very poor. In this connection, theinventors of the present invention have investigated the encapsulationby LiPO₃ glass. Even where a complete coverage of the particle isaccomplished, a significant improvement of air-stability could not bemade and electrochemical properties were poor.

Therefore, there is a strong need for the development of a LiNiO₂-basedcathode active material that can be produced at a low cost frominexpensive precursors, and which show improved properties such as lowswelling when applied to commercial lithium secondary batteries,improved chemical stability and improved structural safety, and highcapacity.

SUMMARY OF THE INVENTION

Therefore, the present invention is provided herewith in view of theabove problems and other technical problems that have yet to beresolved.

As a result of a variety of extensive and intensive studies andexperiments and in view of the problems as described above, theinventors of the present invention provide herewith a cathode activematerial, containing a lithium mixed transition metal oxide having agiven composition, prepared by a solid-state reaction of Li₂CO₃ with amixed transition metal precursor under an oxygen-deficient atmosphere,and being substantially free of Li₂CO₃, exhibits a high capacity,excellent cycle characteristics, significantly improved storage andhigh-temperature stability, and can be produced with low productioncosts and improved production efficiency. The present invention has beencompleted based on these findings.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and other advantages of thepresent invention will be more clearly understood from the followingdetailed description taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a schematic view showing a crystal structure of a conventionalNi-based lithium transition metal oxide;

FIG. 2 is a schematic view showing a crystal structure of a Ni-basedlithium mixed transition metal oxide prepared by a method according toone embodiment;

FIGS. 3 and 4 are graphs showing a preferred composition range of aNi-based lithium mixed transition metal oxide prepared by a methodaccording to one embodiment;

FIG. 5 is an FESEM (Field Emission Scanning Electron Microscope) image(x 2,000) showing LiNiMO₂ according to Example 1. 5A: 850° C.; 5B: 900°C.; 5C: 950° C.; and 5D: 1,000° C.;

FIG. 6 is an FESEM image showing commercial LiMO₂ (M=Ni_(0.8)Co_(0.2))according to Comparative Example 1. 6A: FESEM image of a sample asreceived, and 6B: FESEM image of a sample after heating to 850° C. inair;

FIG. 7 is an FESEM image showing the standard pH titration curve ofcommercial high-Ni LiNiO₂ according to Comparative Example 2. A: Sampleas received, B: After heating of a sample to 800° C. under an oxygenatmosphere, and C: Copy of A;

FIG. 8 is a graph showing a pH titration curve of a sample according toComparative Example 3 during storage of the sample in a wet chamber. A:Sample as received, B: After storage of a sample for 17 hrs, and C:After storage of a sample for 3 days;

FIG. 9 is a graph showing a pH titration curve of a sample according toExample 2 during storage of the sample in a wet chamber. A: Sample asreceived, B: After storage of a sample for 17 hrs, and C: After storageof a sample for 3 days;

FIG. 10 is a graph showing lengths of a-axis and c-axis ofcrystallographic unit cells of samples having different ratios of Li:Min Experimental Example 3;

FIG. 11 is an SEM image of a sample according to Example 4;

FIG. 12 shows the Rietveld refinement on X-ray diffraction patterns of asample according to Example 4;

FIG. 13 is an SEM image (x 5000) of a precursor in Example 5, which isprepared by an inexpensive ammonia-free process and has a low density;

FIG. 14 is a graph showing electrochemical properties of LiNiMO₂according to the present invention in Experimental Example 1. 14A: Graphshowing voltage profiles and rate characteristics at room temperature (1to 7 cycles); 14B: Graph showing cycle stability at 25° C. and 60° C.and a rate of C/5 (3.0 to 4.3V); and 14C: Graph showing dischargeprofiles (at C/10 rate) for Cycle 2 and Cycle 31, obtained duringcycling at 25° C. and 60° C.;

FIG. 15 is a graph showing DSC (differential scanning calorimetry)values for samples of Comparative Examples 3 and 4 in ExperimentalExample 2. A: Commercial Al/Ba-modified LiNiO₂ of Comparative Example 3,and B: Commercial AlPO₄-coated LiNiO₂ of Comparative Example 4;

FIG. 16 is a graph showing DSC values for LiNiMO₂ according to Example 3in Experimental Example 2;

FIG. 17 is a graph showing electrophysical properties of a polymer cellaccording to one embodiment in Experimental Example 3; and

FIG. 18 is a graph showing swelling of a polymer cell duringhigh-temperature storage in Experimental Example 3.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In accordance with an aspect of the present invention, the above can beaccomplished by the provision of a cathode active material comprising alithium mixed transition metal oxide having a composition represented byFormula I below, prepared by a solid-state reaction of Li₂CO₃ and amixed transition metal precursor under an oxygen-deficient atmosphere,and having a Li₂CO₃ content of less than 0.07% by weight of the cathodeactive material as determined by pH titration:Li_(x)M_(y)O₂   (I)

wherein:

M=M′_(1-k)A_(k), wherein M′ is Ni_(1-a-b)(Ni_(1/2)Mn_(1/2))_(a)Co_(b),0.65≦a+b≦0.85 and 0.1≦b≦0.4;

A is a dopant;

0≦k<0.05; and

x+y≈2 and 0.95≦x≦1.05.

Therefore, owing to a low Li₂CO₃ content of less than 0.07% by weight,the cathode active material comprising a high-Ni lithium mixedtransition metal oxide having a the above composition so provided inaccordance with the present invention has excellent sintering andstorage stability, excellent high-temperature stability includingdecreased gas evolution, and a high capacity and excellent cyclecharacteristics due to a stable crystal structure. The cathode activematerial is prepared by a simple solid-state reaction in air, using araw material that is environmentally-friendly, cheap and easy to handle,so the present invention can be applied to industrial-scale productionof the cathode active material, at low production cost and highproduction efficiency.

As used herein, the term “high-Ni” means that a content of nickel ishigh relative to the other transition metals present which constitutethe lithium mixed transition metal oxide, such as nickel, manganese,cobalt, and the like. Hereinafter, where appropriate throughout thespecification, the term “lithium mixed transition metal oxide inaccordance with the present invention” is used interchangeably with theterm “LiNiMO₂”. Therefore, NiM in LiNiMO₂ is a suggestive expressionrepresenting a complex composition of Ni, Mn and Co and a high-Nicontent in Formula I.

The composition of the lithium mixed transition metal oxide shouldsatisfy the following specific requirements as defined in Formula I oras shown in FIG. 3:

(i) Ni_(1-(a+b)()Ni_(1/2)Mn_(1/2))_(a)Co_(b) and 0.65≦a+b≦0.85

(ii) 0.1≦b≦0.4, and

(iii) x+y z≈2 and 0.95≦x≦1.05

Regarding the aforementioned requirement (i), Ni_(1-(a+b)) means acontent of Ni³⁺. Therefore, if a mole fraction of Ni³⁺ exceeds 0.35(a+b<0.65), it is impossible to implement an industrial-scale productionin air, using Li₂CO₃ as a raw material, so the lithium mixed transitionmetal oxide should be produced using LiOH as a raw material under anoxygen atmosphere, thereby presenting a problems associated withdecreased production efficiency and consequently increased productioncosts. On the other hand, if a mole fraction of Ni³⁺ is lower than 0.15(a+b>0.85), the capacity per volume of LiNiMO₂ is not competitive ascompared to LiCoO₂.

With regard to the aforementioned requirement (ii), a content of cobalt(b) is from 0.1 to 0.4. If the content of cobalt is excessively high(b>0.4), the overall cost of a raw material increases due to highcontent of cobalt, and the reversible capacity decreases. On the otherhand, if the content of cobalt is excessively low (b<0.1), it isdifficult to achieve sufficient rate characteristics and a high powderdensity of the battery at the same time.

Meanwhile, taking into consideration both of the above requirements (i)and (ii), the total mole fraction of Ni including Ni²⁺ and Ni³⁺ inLiNiMO₂ of the present invention is specifically a relativelynickel-excess as compared to manganese and cobalt and can be from 0.4 to0.7. If a content of nickel is excessively low, it is difficult toachieve a high capacity. Conversely, if a content of nickel isexcessively high, the safety can be significantly lowered. Inconclusion, the lithium transition metal oxide (LiNiMO₂) exhibits alarge volume capacity and low raw material costs, as compared to lithiumcobalt-based oxides.

Further, if the mole fraction of Ni²⁺ is too high relative to the Nicontent, the cation mixing increases to thereby result in formation ofan excessively stable “rock salt” type structure that is locally andelectrochemically non-reactive, where such a rock salt structure notonly hinders charge/discharge and but also can bring about a decrease ina discharge capacity. On the other hand, if the mole fraction of Ni²⁺ istoo low, this can lead to an increase in the structural instabilitywhich thereby lowers the cycle stability. Therefore, the mole fractionof Ni²⁺ should be appropriately adjusted taking into consideration suchproblems that can occur. Specifically, within the range as shown in FIG.3, the mole fraction of Ni²⁺ can be from 0.05 to 04, based on the totalcontent of Ni.

With regard to the aforementioned condition (iii), if a content oflithium is excessively high, i.e. x>1.05, this may result in a problemof decreased stability during charge/discharge cycling, particularly atT=60° C. and a high voltage (U=4.35 V). On the other hand, if a contentof lithium is excessively low, i.e. x<0.95, this may result in poor ratecharacteristics and a decreased reversible capacity.

In an embodiment, LiNiMO₂ may further comprise trace amounts of dopants.Examples of the dopants may include aluminum, titanium, magnesium andthe like, which are incorporated into the crystal structure. Further,other dopants, such as B, Ca, Zr, F, P, Bi, Al, Mg, Zn, Sr, Ga, In, Ge,and Sn, may be included via the grain boundary accumulation or surfacecoating of the dopants without being incorporated into the crystalstructure. These dopants are included in amounts sufficient to increasethe safety, capacity and overcharge stability of the battery while notcausing a significant decrease in the reversible capacity. Therefore, acontent of the dopant is less than 5% by mole (k<0.05), as defined inFormula I. In addition, the dopants may be specifically added in anamount of <1% by mole, within a range that can improve the stabilitywithout causing deterioration of the reversible capacity.

Typically, Ni-based lithium mixed transition metal oxides contain largeamounts of water-soluble bases such as lithium oxides, lithium sulfates,lithium carbonates, and the like. These water-soluble bases may bebases, such as Li₂CO₃ and LiOH, present in LiNiMO₂, or otherwise can bebases produced by ion exchange (H⁺ (water) ← → Li⁺ (surface, an outersurface of the bulk)), performed at the surface of LiNiMO₂. The bases ofthe latter case are usually present at a negligible level.

The former water-soluble bases may be formed due to the presence ofunreacted lithium raw materials primarily upon sintering. This isbecause as production of conventional Ni-based lithium mixed transitionmetal oxides involves an addition of relatively large amounts of lithiumand a low-temperature sintering process so as to prevent thedisintegration of a layered crystal structure, the resulting particleshave relatively large amounts of grain boundaries as compared to thecobalt-based oxides, and a sufficient reaction of lithium ions is notrealized upon sintering.

In addition, even when an initial amount of Li₂CO₃ is low, Li₂CO₃ mayalso be produced during fabrication of the battery or storage ofelectrode active materials. These water-soluble bases react withelectrolytes in the battery to thereby cause gas evolution and batteryswelling, which consequently result in severe deterioration of thehigh-temperature safety.

On the other hand, since the cathode active material in accordance withthe present invention, as defined above, stably maintains the layeredcrystal structure by a specific composition of transition metal elementsand a reaction atmosphere, despite the use of Li₂CO₃ as a raw material,it is possible to carry out the sintering process at a high-temperature,thereby resulting in small amounts of grain boundaries. In addition, asretention of unreacted lithium on surfaces of particles is prevented,the particle surfaces are substantially free of water-soluble bases suchas lithium carbonates, lithium sulfates, and the like. Accordingly, thepresent invention is characterized in that Li₂CO₃ is contained in atrace amount of less than 0.07% by weight of the cathode activematerial.

In the present invention, the content of Li₂CO₃ includes all of Li₂CO₃remaining upon production of the lithium mixed transition metal oxide,or Li₂CO₃ produced during fabrication of the battery or storage ofelectrode active materials.

The content of Li₂CO₃ refers to an extent that upon titration of 200 mLof a solution containing a cathode active material powder with an acidtitrant solution, e.g., 0.1M HCl, the acid titrant solution used toreach a pH of less than 5 is specifically an amount of less than 20 mL,more specifically less than 10 mL. Herein, 200 mL of the aforementionedsolution contains substantially all kinds of the water-soluble bases inthe cathode active material, and is prepared by repeatedly soaking anddecanting 10 g of the cathode active material with water. There are nosignificant influences of parameters such as a total soaking time of thecathode active material powder in water on the amount of water-solublebase extracted.

Therefore, the content of Li₂CO₃ can be determined in terms of an amountof acid solution titrant (e.g., HCl) used to reach pH of less than 5,according to the following method. To accomplish this, 5 g of a cathodeactive material powder is added to 25 mL of water, followed by briefstirring to effect a soaking process. About 20 mL of the clear solutionis separated from the powder after soaking by decanting, and theseparated solutions are pooled. Again, about 20 mL of water is added tothe powder and the resulting mixture is stirred, followed by decantingand pooling. The soaking and decanting are repeated at least 5 times. Inthis manner, a total of 100 mL of the clear solution containingwater-soluble bases is pooled. In another exemplary embodiment, thisprocess can be scaled such that 10 g of the cathode active material isextracted with water by the above soaking and decanting process at least5 times to provide a total of 200 mL of a solution containing thewater-soluble base. A 0.1M HCl solution is added to the thus-pooledsolution, followed by pH titration with stirring. The pH is recorded asa function of time. Experiments are terminated when the pH reaches avalue of less than 3, and the flow rate may be selected such thattitration takes about 20 to about 30 min.

One of important features of the present invention is that a desiredcathode active material is prepared by a solid-state reaction of Li₂CO₃and a mixed transition metal precursor under an oxygen-deficientatmosphere.

In this way, it was found through various experiments conducted by theinventors of the present invention that when conventional high-nickelLiMO₂ is sintered in air containing a trace amount of CO₂, LiMO₂decomposes with a decrease of Ni³⁺ as shown in the following reactionbelow, during which amounts of Li₂CO₃ impurities increase.LiM ³⁺O₂+CO₂ →a Li_(1-x) M _(1+x1) ^(3+,2+)O₂ +b Li₂CO₃ +cO₂

This is believed to be due to that the decomposition of some Ni³⁺ intoNi²⁺ upon sintering results in destabilization of the crystal structure,which consequently leads to an oxide form having excessive cationmixing, i.e. Li-deficient form of Li_(1-a)Ni_(1+a)O₂ having transitionmetal cations misplaced on lithium sites of the crystal structure, andlithium ions, released from partial collapse of the crystal structure,react with CO₂ in air.

The conventional methods of preparing high-nickel LiMO₂ thus suffer fromthe use of Li₂CO₃ as a raw material, which brings about the evolution ofCO₂ due to decomposition of Li₂CO₃, and which then thermodynamicallyhinders further decomposition of Li₂CO₃ necessary for the reaction evenat a low partial pressure of CO₂, consequently resulting in no furtherprogression of the reaction. In addition, excessive addition of Li₂CO₃is accompanied by a problem of residual Li₂CO₃ after the reaction.

Therefore, in order to prevent such problems associated with thelithium-deficiency and cation mixing and in order to increase a relativeamount of Ni³⁺ increased over that of the conventional methods, theproduction reaction for lithium mixed transition metal oxides could becarried out using an excessive amount of LiOH.H₂O as a lithium source,with a ratio of M(OH)₂ and Li of 1:1.05 to 1.15 (M(OH)₂:Li-compound)under a high-oxygen atmosphere.

However, LiOH.H₂O (technical grade) contains primarily >1% by weightLi₂CO₃ impurities that are not decomposed or removed during thesintering process under an oxygen atmosphere and therefore remain in thefinal product. Further, an excess of the residual Li₂CO₃ accelerates theelectrolyte decomposition to thereby result in the evolution of gas.Therefore, the conventional method suffered from various problems suchas disintegration of secondary particles into single primarycrystallites, lowered storage stability, and deterioration of thehigh-temperature safety resulting from the gas evolution due to thereaction of the residual Li₂CO₃ with the electrolyte in the battery.

Further, the lithium mixed transition metal oxide prepared by aconventional method has a layered crystal structure as shown in FIG. 1,and desertion of lithium ions from the reversible lithium layers in thecharged state brings about swelling and destabilization of the crystalstructure due to the repulsive force between oxygen atoms in the MOlayers (mixed-transition metal oxide layers), thus suffering from theproblems associated with sharp decreases in the capacity and cyclecharacteristics, resulting from changes in the crystal structure due torepeated charge/discharge cycles.

As a result of a variety of extensive and intensive studies andexperiments, the inventors of the present invention discovered that whenthe lithium mixed transition metal oxide is prepared by a solid-statereaction of Li₂CO₃ with the mixed transition metal precursor under anoxygen-deficient atmosphere, it is possible to produce a cathode activematerial containing the lithium mixed transition metal oxidesubstantially free of Li₂CO₃, i.e., having a Li₂CO₃ content of less than0.07% by weight, of the cathode active material as determined by pHtitration. In a specific embodiment, the Li₂CO₃ content is less than0.05% by weight of the cathode active material as determined by pHtitration. In a more specific embodiment, the Li₂CO₃ content is lessthan 0.035% by weight of the cathode active material as determined by pHtitration. In another embodiment, a cathode active material comprising acomparative lithium transition metal oxide that is not prepared by thesolid-state reaction of Li₂CO₃ and a mixed transition metal precursorunder an oxygen-deficient atmosphere, has a Li₂CO₃ content of greaterthan or equal to 0.07% by weight of the cathode active material.

Specifically, under the oxygen-deficient atmosphere, desorption of someoxygen atoms takes place from the MO layers, which leads to a decreasein an oxidation number of Ni, thereby increasing amounts of Ni²⁺ ions.As a result, a portion of the Ni²⁺ ions are inserted into the reversiblelithium layers, as shown in FIG. 2. However, contrary to conventionallyknown or accepted ideas in the related art thatintercalation/deintercalation of lithium ions will be hindered due tosuch insertion of Ni²⁺ ions into the reversible lithium layers, aninsertion of an effective amount of Ni²⁺ ions prevents destabilizationof the crystal structure that may occur due to the repulsive forcebetween oxygen atoms in the MO layers, upon charge. As used herein, “aneffective amount of Ni²⁺ ions”, can include about 3 to about 7 molepercent of the total amount of Ni ions present. Therefore, stabilizationof the crystal structure is achieved to result in no occurrence offurther structural collapse by oxygen desorption. Further, it isbelieved that the lifespan characteristics and safety are simultaneouslyimproved, due to no further formation of Ni²⁺ ions with maintenance ofthe oxidation number of Ni ions inserted into the reversible lithiumlayers, even when lithium ions are released during a charge process.Hence, it can be said that such a concept of the present invention is aremarkable one which is completely opposite to and overthrows theconventional idea.

Thus, the present invention can fundamentally prevent the problems thatmay occur due to the presence of the residual Li₂CO₃ in the finalproduct (i.e., the cathode active material), and provides a highlyeconomical process by performing the production reaction using arelatively small amount of inexpensive Li₂CO₃ as a reactant and anoxygen-deficient atmosphere such as air. Further, the sintering andstorage stabilities are excellent due to the stability of the crystalstructure, and thereby the battery capacity and cycle characteristicscan be significantly improved simultaneously with a desired level ofrate characteristics.

However, under an atmosphere with excessive oxygen-deficiency, anexcessive amount of Ni²⁺ ions transfer to the reversible lithium layersduring the synthesis process, thereby resulting in hindrance of theintercalation/deintercalation of lithium ions, and therefore theperformance of the battery cannot be exerted sufficiently. On the otherhand, if the oxygen concentration is excessively high, the desiredamount of Ni²⁺ cannot be inserted into the reversible lithium layers.Taking into consideration such problems, the synthetic reaction may becarried out under an atmosphere with an oxygen concentration ofspecifically 10% to 50% by volume, and more specifically 10% to 30% byvolume. In an exemplary embodiment, he reaction can be carried out underan air atmosphere.

Another feature of the present invention is that raw materials producedby an inexpensive or economical process and being easy to handle can beused, and particularly Li₂CO₃ which is difficult to employ in the priorart can be used itself as a lithium source.

As an added amount of Li₂CO₃ as the lithium source decreases, that is, amolar ratio (Li/M) of lithium to the mixed transition metal source (M)decreases, an amount of Ni inserted into the MO layers graduallyincreases. Therefore, if excessive amounts of Ni ions are inserted intothe reversible lithium layers, the movement of Li⁺ ions duringcharge/discharge processes is hampered, which thereby leads to problemsassociated with a decrease in the capacity or deterioration of the ratecharacteristics. On the other hand, if an added amount of Li₂CO₃ isexcessively large, that is, the Li/M molar ratio is excessively high,the amount of Ni inserted into the reversible lithium layers isexcessively low, which may undesirably lead to structural instability,thereby presenting decreased safety of the battery and poor lifespancharacteristics. Further, at a high Li/M value, amounts of unreactedLi₂CO₃ increase to thereby result in a high pH-titration value, i.e.,production of large amounts of impurities, and consequently thehigh-temperature safety may deteriorate.

Therefore, in one embodiment, an added amount of Li₂CO₃ as the lithiumsource can be from 0.95 to 1.04:1 where the ratio of Li₂CO₃:mixedtransition metal raw material is a w/w ratio, based on the weight of themixed transition metal as the other raw material.

As a result, the product is substantially free of impurities due to alack of surplus Li₂CO₃ in the product (the cathode active material) byadding only a stoichiometric amount (i.e., by not adding an excess) ofthe lithium source, so that there are no problems associated withresidual Li₂CO₃, and thereby a relatively small amount of inexpensiveLi₂CO₃ is used to provide a lithium mixed transition metal compound in ahighly economical process.

As the mixed transition metal precursor, M(OH)₂ or MOOH (M is as definedin Formula I) can be used specifically. As used herein, the term “mixed”means that several transition metal elements are well mixed at theatomic level.

In conventional processes, as the mixed transition metal precursors,mixtures of Ni-based transition metal hydroxides are generally employed.However, these materials commonly contain carbonate impurities. This isbecause Ni(OH)₂ is prepared by co-precipitation of a Ni-based salt suchas NiSO₄ with a base such as NaOH in which the technical grade NaOHcontains Na₂CO₃ and the CO₃ ²⁻ anion is more easily inserted into theNi(OH)₂ structure than the OH anion.

Further, in order to increase an energy density of the cathode activematerial, conventional prior art processes employed MOOH having a hightap density of 1.5 to 3.0. However, the use of such a high tap densityprecursor makes it difficult to achieve the incorporation of thereactant (lithium) into the inside of the precursor particles during thesynthetic process, which then lowers the reactivity to thereby result inproduction of large amounts of impurities. Further, for preparation ofMOOH having a high tap density, co-precipitation of MSO₄ and NaOH shouldbe carried out in the presence of excess ammonia as a complexingadditive. However, ammonia in waste water causes environmental problemsand thus is strictly regulated. It is, however, generally not possibleto prepare the mixed oxyhydroxide having high density by an ammonia-freeprocess that is less expensive, is more environmentally friendly and iseasier to carry out than this process.

However, according to the research performed by the inventors of thepresent invention, it was found that even though the mixed transitionmetal precursor prepared by the ammonia-free process exhibits arelatively low tap density, a lithium mixed transition metal oxideprepared using the thus-prepared precursor which has an excellentsintering stability makes it is possible to prepare a mixed transitionmetal oxide having a superior reactivity.

In this way, the cathode active material in accordance with the presentinvention, as discussed hereinbefore, can maintain a well-layeredstructure due to the insertion of some Ni ions into the reversiblelithium layers, thus exhibiting excellent sintering stability.Accordingly, the present invention can employ the mixed transition metalprecursor having a low tap density, as the raw material.

Therefore, since the raw material, i.e., the mixed transition metalprecursor, is environmentally friendly, can be easily prepared at lowproduction costs and also has a large volume of voids between primaryparticles, e.g. a low tap density, it is possible to easily realize theintroduction of the lithium source into the inside of the precursorparticles, thereby improving the reactivity, and it is also possible toprevent production of impurities and reduce an amount of the lithiumsource (Li₂CO₃) to be used, so the method of the present invention ishighly economical.

As used herein, the term “ammonia-free process” means that only NaOHwithout the use of aqueous ammonia is used as a co-precipitating agentin a co-precipitation process of a metal hydroxide. That is, thetransition metal precursor is obtained by dissolving a metal salt suchas MSO₄ and MNO₃ (M is a metal of a composition to be used) in water,and gradually adding a small amount of a precipitating agent NaOH withstirring. The introduction of ammonia lowers the repulsive force betweenparticles to thereby result in densification of co-precipitatedparticles, which then increases a density of particles. However, when itis desired to obtain a hydroxide having a low tap density as in thepresent invention, there is no need to employ ammonia. In addition tothe above-exemplified sulfates and nitrates, other materials may beemployed as the metal salt.

In one specific embodiment, the tap density of the mixed transitionmetal precursor may be from 1.1 to 1.6 g/cm³. If the tap density isexcessively low, a chargeable amount of the active material decreases,so the capacity per volume may be lowered. On the other hand, if the tapdensity is excessively high, the reactivity with the lithium sourcematerial is lowered and therefore impurities may be undesirably formed.

The solid-state reaction includes a sintering process specifically at600 to 1,100° C. for 3 to 20 hours, and more specifically 800 to 1,050°C. for 5 to 15 hours. If the sintering temperature is excessively high,this may lead to non-uniform growth of particles, and reduction of thevolume capacity of the battery due to a decreased amount of particlesthat can be contained per unit area, arising from an excessively largesize of particles. On the other hand, if the sintering temperature isexcessively low, an insufficient reaction leads to the retention of theraw materials in the particles, thereby presenting the risk of damagingthe high-temperature safety of the battery, and it may be difficult tomaintain a stable structure, due to the deterioration of the volumedensity and crystallinity. Further, if the sintering time is too short,it is difficult to obtain a lithium nickel-based oxide having highcrystallinity. On the other hand, if the sintering time is too long,this may undesirably lead to excessively large particle diameter andreduced production efficiency.

Meanwhile, various additional parameters may arise as the process forpreparation of the lithium mixed transition metal oxide is scaled up. Afew grams of samples in a furnace behave very differently from a few kgof samples, because the gas transport kinetics at a low partial pressureis completely different. Specifically, in a small-scale process, Lievaporation occurs and CO₂ transport is fast, whereas in a large-scaleprocess, these processes are retarded. Where the Li evaporation and CO₂transport are retarded, a gas partial pressure in the furnace increases,which in turn hinders further decomposition of Li₂CO₃ necessary for thereaction, consequently resulting in retention of the unreacted Li₂CO₃,and the resulting LiNiMO₂ decomposes to result in the destabilization ofthe crystal structure.

Accordingly, when it is desired to prepare the lithium mixed transitionmetal oxide in accordance with the present invention on a large-scale,the preparation process is specifically carried out under a high rate ofair circulation. As used herein, the term “large scale” means that asample has a size of 5 kg or more because similar behavior is expectedin 100 kg of sample when the process has been correctly scaled-up, i.e.,a similar gas flow (m³/kg of sample) reaches 100 kg of sample.

In order to achieve high air circulation upon the production of thelithium transition metal oxide by the large-scale mass productionprocess, specifically at least 2 m³ (volume at room temperature) of air,and more specifically at least 10 m³ of air, per kg of the final product(active material), i.e., lithium mixed transition metal oxide, may bepumped into or out of a reaction vessel. As such, even when the presentinvention is applied to a large-scale production process, it is possibleto prepare the cathode active material which is substantially free ofimpurities including water-soluble bases.

In an embodiment of the present invention, a heat exchanger may beemployed to minimize energy expenditure upon air circulation bypre-warming the in-flowing air before it enters the reaction vessel,while cooling the out-flowing air.

In a specific example, air flow of 2 m³/kg corresponds to about 1.5 kgof air at 25° C. The heat capacity of air is about 1 kJ/kg°K and thetemperature difference is about 800K. Thus, at least about 0.33 kWh isrequired per kg of the final sample for air heating. Where the air flowis 10 m³, about 2 kWh is then necessary. Thus, the typical additionalenergy cost amounts to about 2 to about 10% of the total cathode salesprice. The additional energy cost can be significantly reduced when theair-exchange is made by using a heat exchanger. In addition, the use ofthe heat exchanger can also reduce the temperature gradient in thereaction vessel. To further decrease the temperature gradient, it isrecommended to provide several air flows into the reaction vesselsimultaneously.

The cathode active material in accordance with the present invention maybe comprised only of the lithium mixed transition metal oxide having theabove-specified composition or, where appropriate, it may be comprisedof the lithium mixed transition metal oxide in conjunction with otherlithium-containing transition metal oxides.

Examples of the lithium-containing transition metal oxides that can beused in the present invention may include, but are not limited to,layered compounds such as lithium cobalt oxide (LiCoO₂) and lithiumnickel oxide (LiNiO₂), or compounds substituted with one or moretransition metals; lithium manganese oxides such as compounds of FormulaLi_(1+y)Mn_(2-y)O₄ (0≦y≦0.33), LiMnO₃, LiMn₂O₃, and LiMnO₂; lithiumcopper oxide (Li₂CuO₂); vanadium oxides such as LiV₃O₈, V₂O₅, andCu₂V₂O₇; Ni-site type lithium nickel oxides of Formula LiNi_(1-y)M_(y)O₂(M=Co, Mn, Al, Cu, Fe, Mg, B, or Ga, and 0.01≦y≦0.3); lithium manganesecomposite oxides of Formula LiMn_(2-y)M_(y)O₂ (M=Co, Ni, Fe, Cr, Zn, orTa, and 0.01≦y≦0.1), or Formula Li₂Mn₃MO₈ (M=Fe, Co, Ni, Cu, or Zn);LiMn₂O₄ wherein a portion of Li is substituted with alkaline earth metalions; disulfide compounds; and Fe₂(MoO₄)₃, LiFe₃O₄, and the like.

In accordance with another aspect of the present invention, there isprovided a lithium secondary battery comprising the aforementionedcathode active material. The lithium secondary battery is generallycomprised of a cathode, an anode, a separator and a lithiumsalt-containing non-aqueous electrolyte. Methods for preparing thelithium secondary battery are well-known in the art and thereforedetailed description thereof will be omitted herein.

In accordance with a further aspect of the present invention, there isprovided a method for determining the total amount of water-soluble basecontained in a cathode active material from the amount of an aqueousacid solution titrant neutralized by pH titration of a solutioncontaining the water-soluble base until the pH of the solution reaches avalue of less than 5, wherein the solution of water-soluble base isprepared by repeatedly soaking and decanting 10 g of the cathode activematerial with water until the resulting solution contains all of thewater-soluble bases in the cathode active material. In an embodiment,the acid solution titrant is 0.1M HCl solution. In another embodiment,the total volume of the solution of water-soluble base is 200 mL

That is, all of the water-soluble bases contained in a cathode activematerial are readily dissolved by repeated soaking and decanting of thecathode active material, so the amount of the water-soluble bases can beprecisely determined in a reproducible manner. Therefore, it is possibleusing this method to predict probable deterioration of high-temperaturesafety or cycle characteristics that may occur due to the presence ofimpurities in the battery fabricated using the cathode active material.Knowledge about the content of the water-soluble bases can be a potentmethod for use in the development of a cathode active material havingsuperior storage stability as disclosed herein.

Upon pH titration with addition of 0.1M HCl, this process is generallynegligible at normal speed (i.e., about 30 min), but is carried out for5 hours or less. This is because deviations of the pH profile may occurin a slow ion-exchange process (H⁺ in the solution ← ! Li⁺ in thepowder). Such deviations of the pH profile would occur mostly at pH ofless than about 5.

Upon only measuring pH, for example, as described in EP 1 317 008 A2,even a small amount of LiOH-type impurities can give a higher pH thanthat obtained for a significantly harmful Li₂CO₃ impurities. Therefore,it is important to measure the pH profile in order to characterize whichsoluble bases are present. Accordingly, in one preferred embodiment, itis possible to understand the properties of the water-soluble bases frompH profile, by recording of the pH profile upon pH titration.

Appropriate modifications may be made with kinds and concentrations ofacids used for pH titration, a reference pH and the like, and it shouldbe understood that those modifications are apparent to those skilled inthe art and fall within the scope of the invention.

EXAMPLES

Now, the present invention will be described in more detail withreference to the following Examples. These examples are provided onlyfor illustrating the present invention and should not be construed aslimiting the scope and spirit of the present invention. For reference,the content of water-soluble bases contained in the powder in theworking examples was measured according to the following method.

Contents and Characterization of Water-Soluble Bases (pH Titration)

First, 5 g of a cathode active material powder was added to 25 mL ofwater, followed by brief stirring. About 20 mL of a clear solution wasseparated after this soaking from the powder by decanting and poolingthe supernatant. Again, about 20 mL of water was added to the powder andthe resulting mixture was soaked and stirred, followed by decanting andpooling. The soaking and decanting were repeated at least 5 times. Inthis manner, a total of 100 mL of the clear solution containingwater-soluble bases was pooled. A 0.1M HCl solution was added to thethus-pooled solution, followed by pH titration with stirring. The pH wasrecorded as a function of time. Experiments were terminated when the pHreached a value of less than about 3, and a flow rate was appropriatelyselected within a range in which titration takes about 20 to about 30min. The content of the water-soluble bases was measured as an amount ofacid that was used until the pH reaches a value of less than about 5.Characterization of water-soluble bases was made from the pH profile.

Example 1

A mixed oxyhydroxide of Formula MOOH(M=Ni_(4/15)(Mn_(1/2)Ni_(1/2))_(8/15)Co_(0.2)) as a mixed transitionmetal precursor and Li₂CO₃ were mixed in a stoichiometric molar ratio(Li:M=1.02:1), and the mixture was sintered in air at temperatures of850 (Ex. 1A), 900 (Ex. 1B), 950 (Ex. 1C), and 1,000° C. (Ex. 1D) for 10hours, to prepare a lithium mixed transition metal oxide. Herein,secondary particles were maintained intact without being collapsed, andthe crystal size increased with an increase in the sinteringtemperature.

X-ray analysis showed that all samples have a well-layered crystalstructure. Further, a unit cell volume did not exhibit a significantchange with an increase in the sintering temperature, thus representingthat there was no significant oxygen-deficiency and no significantincrease of cation mixing, in conjunction with essentially no occurrenceof lithium evaporation.

The crystallographic data for the thus-prepared lithium mixed transitionmetal oxide are given in Table 1 below, and FESEM images thereof areshown in FIG. 5. From these results, it was found that the lithium mixedtransition metal oxide is LiNiMO₂ having a well-layered crystalstructure with the insertion of nickel at a level of 3.9 to 4.5% intothe reversible lithium layer. Further, it was also found that eventhough Li₂CO₃ was used as a raw material and sintering was carried outin air, a sufficient amount of Ni²⁺ ions was inserted into the lithiumlayer, thereby achieving the desired structural stability.

Particularly, Sample B, sintered at 900° C. (Ex. 1B), exhibited a highc:a ratio and therefore excellent crystallinity, a low unit cell volumeand a reasonable cation mixing ratio. As a result, Sample B showed themost excellent electrochemical properties, and a BET surface area ofabout 0.4 to about 0.8 m²/g. TABLE 1 Example 1 (A-D) (A) (B) (C) (D)Sintering temp. 850° C. 900° C. 950° C. 1,000° C. Unit cell vol. 33.902Å³ 33.921 Å³ 33.934 Å³ 33.957 Å³ Normalized c:a ratio 1.0123 1.01221.0120 1.0117 c:a/24{circumflex over ( )}0.5 Cation mixing 4.5% 3.9%4.3% 4.5% (Rietveld refinement)

Comparative Example 1

50 g of a commercial sample having a composition ofLiNi_(0.8)Co_(0.1)Mn_(0.1)O₂ represented by Formula LiNi_(1-x)M_(x)O₂(x=0.3, and M=Mn_(1/3)Ni_(1/3)Co_(1/3)) was heated in air to 750° C.(CEx. 1A), 850° C. (CEx. 1B), 900° C. (CEx. 1C) and 950° C. (CEx. 1D)(10 hrs), respectively.

X-ray analysis was carried out to obtain detailed lattice parameterswith high resolution. Cation mixing was observed by Rietveld refinement,and morphology was analyzed by FESEM. The results thus obtained aregiven in Table 2 below. Referring to Table 2, it can be seen that all ofthe samples heated to a temperature of T≧750° C. (CEx. 1A-D) exhibitedcontinuous degradation of a crystal structure (increased cation mixing,increased lattice constant and decreased c:a ratio). FIG. 6 shows aFESEM image of a commercial sample as received and a FESEM image of thesame sample heated to 850° C. (CEx. 1B) in air; and it can be seen thatthe sample heated to a temperature of T≧850° C. (CEx. 1B-D) exhibitedstructural collapse. This is believed to be due to that Li₂CO₃, formedduring heating in air, melts to thereby segregate particles. TABLE 2Comp. Ex. 1 (A-D) (A) (B) (C) (D) Sintering temp. 750° C. 850° C. 900°C. 950° C. Unit cell vol. 33.902 Å³ 33.920 Å³ 33.934 Å³ 33.957 Å³Normalized c:a ratio 1.0103 1.0100 1.0090 1.0085 c:a/24{circumflex over( )}0.5 Cation mixing 10% 12% 15% 18% (Rietveld refinement)

Therefore, it can be seen that it is impossible to produce aconventional lithium mixed transition metal oxide in the air containingtrace amounts of carbon dioxide, due to thermodynamic limitations. Inaddition, upon producing the lithium mixed transition metal oxideaccording to a conventional method, the use of Li₂CO₃ as a raw materialis accompanied by evolution of CO₂ due to decomposition of Li₂CO₃,thereby leading to thermodynamic hindrance of further decomposition ofLi₂CO₃ necessary for the reaction, consequently resulting in no furtherprogression of the reaction. For these reasons, it was shown that such aconventional method cannot be applied to the practical productionprocess.

Comparative Example 2

The pH titration was carried out at a flow rate of >2 L/min for 400 g ofa commercial sample having a composition of LiNi_(0.8)Co_(0.2)O₂. Theresults thus obtained are given in FIG. 7. In FIG. 7, Curve A (CEx. 2A)represents pH titration for the sample as received, and Curve B (CEx.2B) represents pH titration for the sample heated to 800° C. in a flowof pure oxygen for 24 hours. From the analysis results of pH profiles,it can be seen that the contents of Li₂CO₃ before and after heattreatment were the same therebetween, and there was no reaction ofLi₂CO₃ impurities. That is, it can be seen that the heat treatment underan oxygen atmosphere resulted in no additional production of Li₂CO₃impurities, but Li₂CO₃ impurities present in the particles were notdecomposed. Through slightly increased cation mixing, a slightlydecreased c:a ratio and a slightly decreased unit cell volume from theX-ray analysis results, it was found that the content of Li slightlydecreased in the crystal structure of LiNiO₂ in conjunction with theformation of a small amount of Li₂O. Therefore, it can be seen that itis impossible to prepare a stoichiometric lithium mixed transition metaloxide with no impurities and no lithium-deficiency in a flow of oxygengas or synthetic air.

Comparative Example 3

LiAl_(0.02)Ni_(0.78)Co_(0.2)O₂ containing less than 3% aluminumcompound, as commercially available Al/Ba-modified, high-nickel LiNiO₂,was stored in a wet chamber (90% relative humidity, abbreviated “RH”) at60° C. in air. The pH titration was carried out for a sample prior toexposure to moisture, and samples wet-stored for 17 hrs and 3 days,respectively. The results thus obtained are given in FIG. 8. Referringto FIG. 8, an amount of water-soluble bases was low before storage, butsubstantial amounts of water-soluble bases, primarily comprising Li₂CO₃,were continuously formed upon exposure to air. Therefore, even when aninitial amount of Li₂CO₃ impurities was low, it was revealed that thecommercially available high-nickel LiNiO₂ is not stable in air andtherefore rapidly decomposes at a substantial rate, and substantialamounts of Li₂CO₃ impurities are formed during storage.

Example 2

The pH titration was carried out for a sample of the lithium mixedtransition metal oxide in accordance with Example 2 prior to exposure tomoisture, and samples stored in a wet chamber (90% RH) at 60° C. in airfor 17 hours and 3 days, respectively. The results thus obtained aregiven in FIG. 9.

Upon comparing the lithium mixed transition metal oxide of Example 2(see FIG. 9) with the sample of Comparative Example 3 (see FIG. 8), thesample of Comparative Example 3 (stored for 17 hours) exhibitedconsumption of about 20 mL of HCl, whereas the sample of Example 2(stored for 17 hours) exhibited consumption of 10 mL of HCl, thusshowing an about two-fold decrease in production of the water-solublebases. Further, in 3-day-storage samples, the sample of ComparativeExample 3 exhibited consumption of about 110 mL of HCl, whereas thesample of Example 2 exhibited consumption of 26 mL of HCl, whichcorresponds to an about five-fold decrease in production of thewater-soluble bases. Therefore, it can be seen that the sample ofExample 2 decomposed at a rate about five-fold slower than that of thesample of Comparative Example 3. Then, it can be shown that the lithiummixed transition metal oxide of Example 2 exhibits superior chemicalresistance even when it is exposed to air and moisture.

Comparative Example 4

A high-nickel LiNiO₂ sample having a composition ofLiNi_(0.8)Mn_(0.05)Co_(0.15)O₂, as a commercial sample which wassurface-coated with AlPO₄ followed by gentle heat treatment, wassubjected to pH titration before and after storage in a wet chamber. Asa result of pH titration, 12 mL of 0.1M HCl was consumed per 10 gcathode, an initial content of Li₂CO₃ was low, and the content of Li₂CO₃after storage was slightly lower (80 to 90%) as compared to the sampleof Comparative Example 3, but the content of Li₂CO₃ was higher than thatof Example 2. Consequently, it was found that the aforementioned high-NiLiNiO₂ shows no improvements in the stability against exposure to theair even when it was surface-coated, and also exhibits insignificantimprovements in the electrochemical properties such as the cyclestability and rate characteristics.

Example 3

Samples with different Li:M molar ratios were prepared from MOOH(M=Ni_(4/15)(Mn_(1/2)Ni_(1/2))_(8/15)Co_(0.2)). Li₂CO₃ was used as alithium source. Specifically, 7 samples each of about 50 g with Li:Mratios ranging from 0.925 to 1.12 were prepared by a sintering processin air at a temperature of 910 to 920° C. Then, electrochemicalproperties were tested.

Table 3 below provides the obtained crystallographic data. The unit cellvolume changes smoothly according to the Li:M ratio. FIG. 10 shows itscrystallographic map. All samples are located on a straight line.According to the results of pH titration, the content of soluble baseslightly increased with an increase of the Li:M ratio, but the totalamount thereof was small. Accordingly, the soluble base probablyoriginates from the surface basicity (i.e., is present by an ionexchange mechanism) but not from the dissolution of Li₂CO₃ impurities asobserved in Comparative Example 1.

Therefore, this experiment clearly shows that the lithium mixedtransition metal oxide prepared by the method in accordance with thepresent invention is in the Li stoichiometric range and additional Li isinserted into the crystal structure. In addition, it can be seen thatstoichiometric samples without Li₂CO₃ impurity can be obtained even whenLi₂CO₃ is used as a precursor and the sintering is carried out in air.

That is, as the Li/M molar ratio decreases, the amount of Ni²⁺ ionsinserted into the reversible lithium layer gradually increases.Insertion of excessively large amounts of Ni²⁺ into the reversiblelithium layer hinders the movement of Li⁺ during the charge/dischargeprocess, thereby resulting in decreased capacity or poor ratecharacteristics. On the other hand, if the Li/M molar ratio isexcessively high, the amount of Ni²⁺ inserted into the reversiblelithium layer is too low, which may result in structural instabilityleading to deterioration of the battery safety and lifespancharacteristics. Further, at the high Li/M value, amounts of unreactedLi₂CO₃ increase to thereby result in a high pH-titration value.Therefore, upon considering the performance and safety of the battery,the molar ratio of Li:M is specifically from 0.95 to 1.04 (Samples B, Cand D) to ensure that the value of Ni²⁺ inserted into the lithium layeris from 3 to 7%. TABLE 3 Samples A B C D E F G Li:M ratio (molar) 0.9250.975 1.0 1.025 1.05 1.075 1.125 Unit cell vol. 34.111 Å³ 34.023 Å³33.923 Å³ 33.921 Å³ 33.882 Å³ 33.857 Å³ 33.764 Å³ c:a ratio 1.01161.0117 1.0119 1.0122 1.0122 1.0123 1.0125 Cation mixing 8.8% 6.6% 4.7%4.0% 2.1% 2.5% 1.4% pH 3 3.5 5 9 15 19 25

Example 4

Li₂CO₃ and a mixed oxyhydroxide of Formula MOOH(M=Ni_(4/15)(Mn_(1/2)Ni_(1/2))_(8/15)Co_(0.2)) were introduced into afurnace with an about 20 L chamber and sintered at 920° C. for 10 hours,during which more than 10 m³ of air was pumped into the furnace, therebypreparing about 5 kg of LiNiMO₂ in one batch.

After sintering was complete, a unit cell constant was determined byX-ray analysis, and a unit cell volume was compared with a target value(Sample B of Example 1: 33.921 Å³). ICP analysis found that the molarratio of Li and M is very close to 1.00, and the unit cell volume waswithin the target range. FIG. 11 shows an scanning electron microscope(SEM) image of the thus-prepared cathode active material and FIG. 12shows results of Rietveld refinement. Referring to these drawings, itcan be seen that the sample exhibits high crystallinity and well-layeredstructure, a mole percentage of Ni²⁺ ions inserted into a reversiblelithium layer is 3.97%, and the calculated value and the measured valueof the mole percentage of Ni²⁺ ions is approximately the same.

Meanwhile, upon performing pH titration, less than 10 mL of 0.1M HCl wasconsumed to titrate 10 g of a cathode to achieve a pH of less than 5,which corresponds to a Li₂CO₃ impurity content of less than about 0.035wt %. Hence, these results show that it is possible to achieve massproduction of substantially Li₂CO₃-free LiNiMO₂ having a stable crystalstructure from the mixed oxyhydroxide and Li₂CO₃ by a solid-statereaction.

Example 5

More than 1 kg of MOOH (M=Ni_(4/15)(Mn_(1/2)Ni_(1/2))_(8/15)Co_(0.2))was prepared by ammonia-free coprecipitation of MSO₄ and NaOH at 80° C.under the pH-adjustment condition. FIG. 13 shows an SEM micrograph ofthe thus-prepared precursor hydroxide. The aforementioned MOOH exhibiteda narrow particle diameter distribution, and a tap density of about 1.2g/cm³. A lithium mixed transition metal oxide was prepared using MOOH asa precursor. Sintering was carried out at 930° C. The lithium mixedtransition metal oxide prepared using such a precursor did not exhibitthe disintegration of particles as shown in Comparative Example 2.Therefore, from the excellent sintering stability of LiMO₂, it can beseen that LiMO₂ can be prepared from the mixed oxyhydroxide having a lowtap density.

Experimental Example 1 Test of Electrochemical Properties

Coin cells were fabricated using the lithium mixed transition metaloxide of Examples 3 and 5, and LiNiMO₂ of Comparative Examples 2 to 4(M=(Ni_(1/2)Mn_(1/2))_(1-x)Co_(x) and x=0.17 (Comparative Example 5) andx=0.33 (Comparative Example 6), respectively, as a cathode, and alithium metal as an anode. Electrochemical properties of thethus-fabricated coin cells were tested. Cycling was carried outprimarily at 25° C. and 60° C., a charge rate of C/5 and a dischargerate of C/5 (1 C=150 mA/g) from 3 to 4.3 V.

Experimental results of the electrochemical properties for the coincells of Comparative Examples 2 to 4 are given in Table 4 below.Referring to Table 4, the cycle stability was poor with the exception ofComparative Example 3 (Sample B). It is believed that ComparativeExample 4 (Sample C) exhibits the poor cycle stability due to thelithium-deficiency of the surface. Whereas, even though ComparativeExample 2 (Sample A) and Comparative Example 3 (Sample B) were notlithium-deficient, only Comparative Example 3 (Sample B) exhibited a lowcontent of Li₂CO₃. The presence of such Li₂CO₃ may lead to gas evolutionand gradual degradation of the performance (at 4.3 V, Li₂CO₃ slowlydecomposes with the collapse of crystals). That is, there are nonickel-based active materials meeting both the excellent cycle stabilityand the low-impurity content, and therefore it can be shown that nocommercial product is available in which the nickel-based activematerial has excellent cycle stability and high stability againstexposure to air, in conjunction with a low level of Li₂CO₃ impuritiesand low production costs. TABLE 4 Sample (A) Sample (B) Sample (C)LiNi_(0.8)Co_(0.2)O₂ Al/Ba-modified AlPO₄-coated Substrate Comp. Ex. 2Comp. Ex. 3 Comp. Ex. 4 Stoichiometric Stoichiometric StoichiometricLi-deficient at Li:M surfaces Li₂CO₃ High High Low impurities Capacityat 193, 175 mAh/g 195, 175 mAh/g 185, 155 mAh/g 25° C. C/10, C/1Capacity loss 30% per 100 11% per 100 >30% per cycles cycles 100 cycles

On the other hand, the cells of Comparative Examples 5 and 6 exhibited acrystallographic density of 4.7 and 4.76 g/cm³, respectively, which werealmost the same, and showed a discharge capacity of 157 to 159 mAh/g ata C/10 rate (3 to 4.3 V). Upon comparing with LiCoO₂ having acrystallographic density of 5.04 g/cm³ and a discharge capacity of 157mAh/g, a volume capacity of the cell of Comparative Example 5 is equalto a 93% level of LiCoO₂, and the cell of Comparative Example 6 exhibitsa crystallographic density corresponding to a 94% level of LiCoO₂.Therefore, it can be seen that a low content of Ni results in a poorvolume capacity.

Table 5 below summarizes electrochemical results of coin cells usingLiNiMO₂ in accordance with Example 3 as a cathode, and FIG. 14 depictsvoltage profiles, discharge curves and cycle stability. Acrystallographic density of LiNiMO₂ in accordance with Example 3 was4.74 g/cm³ (cf. LiCoO₂: 5.05 g/cm³). A discharge capacity was more than170 mAh/g (cf. LiCoO₂: 157 mAh/g) at C/20, thus representing that thevolume capacity of LiNiMO₂ was much improved as compared to LiCoO₂.Electrochemical properties of LiNiMO₂ in accordance with Example 5 weresimilar to those of Example 3. TABLE 5 Capacity retention after Primary100 cycles charge (extrapolated) capacity C/5-C/5 cycle, 3.0-4.3 V,Discharge capacity 3.0-4.3 V C/10 25° C., 25° C. 60° C. — C/1 25° C.,C/20 60° C., C/20 >96% >90% >190 mAh/g 152 mA/g 173 mAh/g 185 mAh/g

Experimental Example 2 Determination of Thermal Stability

In order to examine the thermal stability for the lithium mixedtransition metal oxide of Example 3 and LiNiMO₂ in accordance withComparative Examples 3 and 4, DSC analysis was carried out. Thethus-obtained results are given in FIGS. 15 and 16. For this purpose,coin cells (anode: lithium metal) were charged to 4.3 V, disassembled,and inserted into hermetically sealed DSC cans, followed by injection ofan electrolyte. A total weight of the cathode was from about 50 to about60 mg, A total weight of the electrolyte was approximately the same.Therefore, an exothermic reaction is strongly cathode-limited. The DSCmeasurement was carried out at a heating rate of 0.5 K/min.

As a result, Comparative Example 3 (Al/Ba-modified LiNiO₂) andComparative Example 4 (AlPO₄-coated LiNiO₂) showed the initiation of astrong exothermic reaction at a relatively low temperature.Particularly, Comparative Example 3 exhibited a heat evolution thatexceeds the limit of the device. The total accumulation of heatgeneration was large, i.e. well above 2,000 kJ/g, thus indicating a lowthermal stability (see FIG. 15).

Meanwhile, LiNiMO₂ of Example 3 in accordance with the present inventionexhibited a low total heat evolution, and the initiation of anexothermic reaction at a relatively high temperature as compared toComparative Examples 3 and 4 (see FIG. 16). Therefore, it can be seenthat the thermal stability of LiNiMO₂ in accordance with the presentinvention is excellent.

Experimental Example 3 Test of Electrochemical Properties of PolymerCells with Application of Lithium Mixed Transition Metal Oxide

Using the lithium mixed transition metal oxide of Example 3 as a cathodeactive material, a pilot plant polymer cell of 383562 type wasfabricated. For this purpose, the cathode was mixed with 17% by weightLiCoO₂, and the cathode slurry was an NMP/PVDF-based slurry. Noadditives for the purpose of preventing gelation were added. The anodewas a mesocarbon microbead (MCMB) anode. The electrolyte was a standardcommercial electrolyte free of additives known to reduce excessiveswelling. Experiments were carried out at 60° C. and charge anddischarge rates of C/5. A charge voltage was from 3.0 to 4.3 V.

FIG. 17 shows the cycle stability of the battery of the presentinvention (0.8 C charge, 1C discharge, 3 to 4 V, 2 V) at 25° C. Anexceptional cycle stability (91% at C/1 rate after 300 cycles) wasachieved at room temperature. The impedance build up was low. Also, thegas evolution during storage was measured. The results thus obtained aregiven in FIG. 18. During a 4 h-90° C. fully charged (4.2 V) storage, avery small amount of gas was evolved and only a small increase ofthickness was observed. The increase of thickness was within or lessthan the value expected for good LiCoO₂ cathodes tested in similar cellsunder similar conditions. Therefore, it can be seen that LiNiMO₂prepared by the method in accordance with the present invention exhibitsvery high stability and chemical resistance.

Example 6

A mixed hydroxide of Formula MOOH(M=Ni_(4/15)(Mn_(1/2)Ni_(1/2))_(8/15)Co_(0.2)) as a mixed transitionmetal precursor and Li₂CO₃ were mixed in a molar ratio of Li:M=1.01:1,and the mixture was sintered in air at 900° C. for 10 hours, therebypreparing 50 g of a lithium mixed transition metal oxide having acomposition of LiNi_(0.53)Co_(0.2)Mn_(0.27)O₂.

X-ray analysis was carried out to obtain detailed lattice parameterswith high resolution. Cation mixing was observed by Rietveld refinement.The results thus obtained are given in Table 6 below.

Comparative Example 7

A lithium mixed transition metal oxide was prepared in the same manneras in Example 6, except that a molar ratio of Li:M was set to 1:1 andsintering was carried out under an O₂ atmosphere. Then, X-ray analysiswas carried out and the cation mixing was observed. The results thusobtained are given in Table 6 below. TABLE 6 Ex. 4 Comp. Ex. 7 Li:M1.01:1 1:1 Unit cell vol. (Å³) 33.921 33.798 Normalized c:a ratio 1.01221.0124 c:a/24{circumflex over ( )}0.5 Cation mixing 4.6% 1.5%

As can be seen from Table 6, the lithium mixed transition metal oxide ofExample 6 in accordance with the present invention exhibited a largerunit cell volume and a smaller c:a ratio, as compared to that ofComparative Example 7. Therefore, it can be seen that the lithium mixedtransition metal oxide of Comparative Example 7 exhibited an excessivelylow cation mixing ratio due to the heat treatment under the oxygenatmosphere. This case suffers from deterioration of the structuralstability. That is, it can be seen that the heat treatment under theoxygen atmosphere resulted in the development of a layered structure dueto excessively low cation mixing, but migration of Ni²⁺ ions washindered to an extent that the cycle stability of the battery isarrested.

Example 7

A lithium mixed transition metal oxide having a composition ofLiNi_(0.4)Co_(0.3)Mn_(0.3)O₂ was prepared in the same manner as inExample 6, except that a mixed hydroxide of Formula MOOH(M=Ni_(1/10)(Mn_(1/2)Ni_(1/2))_(6/10)Co_(0.3)) was used as a mixedtransition metal precursor, and the mixed hydroxide and Li₂CO₃ weremixed in a molar ratio of Li:M=1:1. The cation mixing was observed byX-ray analysis and Rietveld refinement. The results thus obtained aregiven in Table 7 below. TABLE 7 Li:M 1:1 Unit cell vol. 33.895 Å³Normalized c:a ratio 1.0123 c:a/24{circumflex over ( )}0.5 Cation mixing3% Capacity (mAh/g) 155

Example 8

A lithium mixed transition metal oxide having a composition ofLiNi_(0.65)Co_(0.2)Mn_(0.15)O₂ was prepared in the same manner as inExample 6, except that a mixed hydroxide of Formula MOOH(M=Ni_(5/10)(Mn_(1/2)Ni_(1/2))_(3/10)Co_(0.2)) was used as a mixedtransition metal precursor, and the mixed hydroxide and Li₂CO₃ weremixed in a molar ratio of Li:M=1:1. The cation mixing was observed byX-ray analysis and Rietveld refinement. The results thus obtained aregiven in Table 8 below. TABLE 8 Li:M 1:1 Unit cell vol. 34.025 Å³Normalized c:a ratio 1.0107 c:a/24{circumflex over ( )}0.5 Cation mixing7% Capacity (mAh/g) 172

From the results shown in Tables 7 and 8, it can be seen that thelithium mixed transition metal oxide in accordance with the presentinvention provides desired effects, as discussed hereinbefore, in agiven range.

INDUSTRIAL APPLICABILITY

As apparent from the above description, a cathode active material inaccordance with the present invention comprises a lithium mixedtransition metal oxide having a given composition, prepared by asolid-state reaction of Li₂CO₃ with a mixed transition metal precursorunder an oxygen-deficient atmosphere, and has a Li₂CO₃ content of lessthan 0.07% by weight of the cathode active material as determined by pHtitration. Therefore, the thus-prepared cathode active material exhibitsexcellent high-temperature stability and stable crystal structure,thereby providing a high capacity and excellent cycle stability, andalso can be produced by an environmentally friendly method with lowproduction costs and high production efficiency.

Although the preferred embodiments of the present invention have beendisclosed for illustrative purposes, those skilled in the art willappreciate that various modifications, additions and substitutions arepossible, without departing from the scope and spirit of the inventionas disclosed in the accompanying claims.

1. A cathode active material comprising a lithium mixed transition metaloxide having a composition represented by Formula I below, wherein thelithium mixed transition metal oxide is prepared by a solid-statereaction of Li₂CO₃ and a mixed transition metal precursor under anoxygen-deficient atmosphere, and having a Li₂CO₃ content of less than0.07% by weight of the cathode active material as determined by pHtitration:Li_(x)M_(y)O₂   (I) wherein: M=M′_(1-k)A_(k), wherein M′ isNi_(1-a-b)(Ni_(1/2)Mn_(1/2))_(a)Co_(b), 0.65≦a+b≦0.85 and 0.1≦b≦0.4; Ais a dopant; 0≦k<0.05; and x+y≈2 and 0.95≦x≦1.05.
 2. The cathode activematerial according to claim 1, wherein Li₂CO₃ is contained in an amountsuch that less than 20 mL of a 0.1M HCl titrant solution is added duringpH titration of a solution of water-soluble bases extracted from thecathode active material to reach a value of less than 5, wherein thesolution of water-soluble bases is prepared by repeatedly soaking anddecanting 10 g of the cathode active material with water such that theresulting solution contains all of the water-soluble bases in thecathode active material, and wherein the total volume of solution ofwater-soluble base is 200 mL.
 3. The cathode active material accordingto claim 2, wherein the amount of the 0.1M HCl solution added during pHtitration to reach the pH of less than 5 is less than 10 mL.
 4. Thecathode active material according to claim 1, wherein theoxygen-deficient atmosphere has an oxygen concentration of 10 to 50% byvolume.
 5. The cathode active material according to claim 4, wherein theoxygen concentration is from 10% to 30% by volume.
 6. The cathode activematerial according to claim 4, wherein the oxygen-deficient atmosphereis an air atmosphere.
 7. The cathode active material according to claim1, wherein a mixing ratio of Li₂CO₃ and the mixed transition metalprecursor for the solid-state reaction is from 0.95 to 1.04:1, whereinthe ratio of Li₂CO₃:mixed transition metal precursor is by w/w.
 8. Thecathode active material according to claim 1, wherein the mixedtransition metal precursor is one or more selected from the groupconsisting of M(OH)₂ and MOOH wherein M is as defined in Formula I. 9.The cathode active material according to claim 8, wherein the mixedtransition metal precursor is MOOH, and is prepared by an ammonia-freeprocess.
 10. The cathode active material according to claim 1, whereinthe mixed transition metal precursor has a tap density of 1.1 to 1.6g/cm³.
 11. The cathode active material according to claim 1, wherein thesolid-state reaction includes a sintering process at 600 to 1100° C. for3 to 20 hours.
 12. The cathode active material according to claim 11,wherein an amount of air exceeding 2 m³/kg LiMO₂ during the sinteringprocess is supplied to a reaction vessel equipped with a heat exchangerfor pre-warming of the air.
 13. The cathode active material according toclaim 1, wherein the lithium mixed transition metal oxide is prepared bya large-scale process under a high rate of air circulation.
 14. Thecathode active material according to claim 13, wherein for the high rateof air circulation during the sintering process, at least 2 m³ of air atroom temperature per 1 kg of the final lithium mixed transition metaloxide is pumped into or out of the reaction vessel.
 15. The cathodeactive material according to claim 14, wherein at least 10 m³ of air atroom temperature per 1 kg of the final lithium mixed transition metaloxide is pumped into or out of the reaction vessel.
 16. The cathodeactive material according to claim 12, wherein the heat exchangerpre-warms the in-flowing air before the in-flowing air enters thereaction vessel, while cooling the out-flowing air.
 17. A lithiumsecondary battery comprising the cathode active material of claim
 1. 18.A method for determining an amount of water-soluble base contained in acathode active material comprises determining an amount of a 0.1 M HCltitrant solution neutralized by pH titration of a solution containingthe water-soluble base until the pH of the solution reaches a value ofless than 5, wherein the solution containing the water-soluble base isprepared by repeatedly soaking and decanting 10 g of the cathode activematerial with water until the resulting solution contains all of thewater-soluble base in the cathode active material, and the total volumeof the solution of water-soluble base is 200 mL.
 19. The methodaccording to claim 18, wherein pH titration with an addition of 0.1M HClis carried out for 5 hours or less.
 20. The cathode active material ofclaim 1, wherein a comparative lithium transition metal oxide that isnot prepared by the solid-state reaction of Li₂CO₃ and a mixedtransition metal precursor under an oxygen-deficient atmosphere, has aLi₂CO₃ content of greater than or equal to 0.07% as determined by pHtitration.